One xform performed by LegalizeDAG is transformation of "store of fp" to "store of...

One xform performed by LegalizeDAG is transformation of "store of fp" to "store of int".
Make two changes:
1) only xform "store of f32" if i32 is a legal type for the target.
2) only xform "store of f64" if either i64 or i32 are legal for the target.
3) if i64 isn't legal, manually lower to 2 stores of i32 instead of letting a
   later pass of legalize do it.  This is ugly, but helps future changes I'm
   about to commit.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@42980 91177308-0d34-0410-b5e6-96231b3b80d8